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Moons of Mars
'' on March 23 2008.]] Mars has two tiny moons, Phobos and Deimos, which are thought to be captured asteroids. Both satellites were discovered in 1877 by Asaph Hall, and are named after the characters Phobos (panic/fear) and Deimos (terror/dread) who, in Greek mythology, accompanied their father Ares, god of war, into battle. Ares was known as Mars to the Romans. History The discovery of the two moons of Mars, Phobos and Deimos, occurred in 1877 when American astronomer Asaph Hall, Sr. identified them after a long search, although their existence had been speculated before. Early speculations The possibility of Martian moons had been speculated before Hall's discovery. The astronomer Johannes Kepler (1571-1630) even predicted their number correctly, although with faulty logic: he wrote that since Jupiter had four known moons and Earth had one, it was only natural that Mars should have two.MathPages - Galileo's Anagrams and the Moons of Mars. Perhaps inspired by Kepler, Jonathan Swift's satire Gulliver's Travels (1726) refers to two moons in part 3, chapter 3 (the "Voyage to Laputa"), in which the astronomers of Laputa are described as having discovered two satellites of Mars orbiting at distances of 3 and 5 Martian diameters, and periods of 10 and 21.5 hours, respectively. This corresponds to the actual orbital distances and periods of Phobos and Deimos of 1.4 and 3.5 Martian diameters, and 7.6 and 30.3 hours, respectively, not remotely close to Swift's fictional satellites. Voltaire's 1750 short story Micromégas, about an alien visitor to Earth, also refers to two moons of Mars. Voltaire was presumably influenced by Swift.William Sheehan, The Planet Mars: A History of Observation and Discovery In recognition of these 'predictions', two craters on Deimos are named Swift and Voltaire. Discovery , discoverer of the two Martian moons.]] Hall discovered Deimos on August 12, 1877 at about 07:48 UTC and Phobos on August 18, 1877, at the US Naval Observatory in Washington, D.C., at about 09:14 GMT (contemporary sources, using the pre-1925 astronomical convention that began the day at noon, give the time of discovery as August 11, 14:40 and August 17 16:06 Washington mean time respectively). Morley, T. A.; [http://adsabs.harvard.edu//full/seri/A+AS./0077//0000220.000.html A Catalogue of Ground-Based Astrometric Observations of the Martian Satellites, 1877-1982], Astronomy and Astrophysics Supplement Series (ISSN 0365-0138), Vol. 77, No. 2 (February 1989), pp. 209–226 (Table II, p. 220: first observation of Phobos on 1877-08-18.38498) At the time, he was deliberately searching for Martian moons. Hall had previously seen what appeared to be a Martian moon on August 10, but due to bad weather, he could not definitively identify them until later. Hall recorded his discovery of Phobos in his notebook as follows: The names, originally spelled Phobus and Deimus, respectively, were suggested by Henry Madan (1838–1901), Science Master of Eton, from Book XV of the Iliad, where Ares summons Fear and Fright. Recent surveys (left) and Phobos (right) in front of Sagittarius, as seen by Mars Exploration Rover Spirit on August 26, 2005 ]] Searches have been conducted for additional satellites. Most recently, Scott S. Sheppard and David C. Jewitt surveyed the Hill sphere of Mars for irregular satellites. The search covered nearly the entire Hill sphere, but scattered light from Mars excluded the inner few arcminutes where the satellites Phobos and Deimos reside. No new satellites were found to an apparent limiting red magnitude of 23.5, which corresponds to radii of about 0.09 km using an albedo of 0.07.Astron. J., 128, 2542-2546 (2004) Characteristics If viewed from the surface of Mars near its equator, full Phobos looks about one third as big as the Earth's full moon from Earth. It has an angular diameter of between 8' (rising) and 12' (overhead). It would look smaller when the observer is further away from the Martian equator, and is completely invisible (always beyond the horizon) from Mars' polar ice caps. Deimos looks more like a bright star or planet for an observer on Mars, only slightly bigger than Venus looks from earth; it has an angular diameter of about 2'. The Sun's angular diameter as seen from Mars, by contrast, is about 21'. Thus there are no total solar eclipses on Mars, as the moons are far too small to completely cover the Sun. On the other hand, total lunar eclipses of Phobos are very common, happening almost every night[http://www.astrobio.net/news/index.php?name=News&file=article&sid=775 Moon Shadows: "Somewhere near the martian equator, Phobos eclipses the sun nearly every day."]. See also Transit of Phobos from Mars and Transit of Deimos from Mars for eclipse-like events. The motions of Phobos and Deimos would appear very different from that of our own Moon. Speedy Phobos rises in the west, sets in the east, and rises again in just eleven hours, while Deimos, being only just outside synchronous orbit, rises as expected in the east but very slowly. Despite its 30 hour orbit, it takes 2.7 days to set in the west as it slowly falls behind the rotation of Mars, and has long again to rise. Both moons are tidally locked, always presenting the same face towards Mars. Since Phobos orbits Mars faster than the planet itself rotates, tidal forces are slowly but steadily decreasing its orbital radius. At some point in the future, when it approaches Mars closely enough (see Roche limit), Phobos will be broken up by these tidal forcesIn 100 million years or so Phobos will likely be shattered by stress caused by the relentless tidal forces, the debris forming a decaying ring around Mars.. Several strings of craters on the Martian surface, inclined further from the equator the older they are, suggest that there may have been other small moons that suffered the fate expected of Phobos, and also that the Martian crust as a whole shifted between these eventsNew Map Provides More Evidence Mars Once Like Earth: "… the new map shows evidence of features, transform faults, that are a "tell-tale" of plate tectonics on Earth.". Deimos, on the other hand, is far enough away that its orbit is being slowly boosted insteadSahife 6: "Deimos orbits far enough away from Mars that it is being slowly pushed farther and farther away from the planet.", as in the case of our own Moon. Orbital details and Deimos.]] See also *Phobos and Deimos in fiction *Natural satellites of Jupiter Saturn Uranus Neptune Pluto *Timeline of discovery of Solar System planets and their natural satellites *Naming of natural satellites Further reading *FIRST INTERNATIONAL CONFERENCE ON THE EXPLORATION OF PHOBOS AND DEIMOS (2007) References Category:Moons of Mars Category:Lists of moons af:Mars se natuurlike satelliete be-x-old:Спадарожнікі Марса bg:Естествени спътници на Марс ca:Satèl·lits de Mart cs:Měsíce Marsu el:Δορυφόροι του Άρη es:Satélites de Marte eo:Naturaj satelitoj de Marso fr:Satellites naturels de Mars gl:Lúas de Marte gu:મંગળનાં ચંદ્રો ko:화성의 위성 id:Bulan Mars it:Satelliti naturali di Marte lv:Marsa pavadoņi hu:A Mars holdjai mk:Сателити на Марс nah:Chīchīlcītlalli īmētz nl:Manen van Mars ja:火星の衛星 nn:Marsmånane pl:Księżyce Marsa ro:Sateliţii naturali ai lui Marte rm:Glinas da Mars ru:Спутники Марса simple:Moons of Mars sk:Mesiace Marsu sl:Marsovi naravni sateliti sr:Природни сателити Марса sv:Mars månar tr:Mars'ın uyduları uk:Супутники Марса ur:مریخ کے چاند zh:火星的衛星